Journal of Colloid and Interface Science 314 (2007) 578–583www.elsevier.com/locate/jcis

Inhibition of acid corrosion of carbon steel using aqueous extractof olive leaves

A.Y. El-Etre

Chemistry Department, Faculty of Science, Benha University, Benha 13815, Egypt

Received 19 April 2007; accepted 25 May 2007

Available online 12 July 2007

Abstract

The inhibitive action of the aqueous extract of olive (Olea europaea L.) leaves toward the corrosion of C-steel in 2 M HCl solution wasinvestigated using weight loss measurements, Tafel polarization, and cyclic voltammetry. It was found that the extract acts as a good corrosioninhibitor for the tested system. The inhibition efficiency increases with increasing extract concentration. The inhibitive action of the extract isdiscussed with a view to adsorption of its components onto the steel surface, making a barrier to mass and charge transfer. The adsorption ofextract components onto the steel surface was found to be a spontaneous process and to follow the Langmuir adsorption isotherm. It was foundalso that such adsorption increases the activation energy of the corrosion process. The results of cyclic voltammetry showed that the presence ofolive extract decreases the charge density in the transpassive region. The inhibition efficiency is greatly reduced as the temperature is increased.© 2007 Elsevier Inc. All rights reserved.

Keywords: Corrosion inhibition; C-steel; Olive; Cyclic voltammetry

1. Introduction

Corrosion inhibitors are chemical compounds usually usedin small concentrations whenever a metal is in contact withan aggressive medium. The presence of such compounds re-tards the corrosion process and keeps its rate to a minimumand thus prevents economic losses due to metallic corrosion.The chemical compounds that could be used for this purposemay be organic or inorganic. However, not just any chemicalcompound can be used as a corrosion inhibitor. There are somerequirements that the compound must fulfill to do so. Regard-ing the chemical structure and chemical behavior, an inorganiccompound must be able to oxidize the metal, forming a pas-sive layer on its surface. On the other hand, a molecule of anorganic compound must have some features that give it theability to act as a corrosion inhibitor. Among these, the mole-cule may have a large structure, double bonds, an active centeror group, etc. These features give the molecule the ability to

E-mail address: aliyousry@hotmail.com.

0021-9797/$ – see front matter © 2007 Elsevier Inc. All rights reserved.doi:10.1016/j.jcis.2007.05.077

cover a large area of a metal surface with a firmly attachedfilm.

Apart from the structural aspects, there are also economicand environmental considerations. Thus, since the whole sub-ject of corrosion is about its destructive economical effect, theused inhibitor must be cheap. Furthermore, due to the recent in-creasing awareness of green chemistry, it must be a nontoxicand environmentally friendly chemical. One of the sources ofthese cheap and clean inhibitors is plants. Plant parts containseveral compounds that satisfy the mentioned criteria. Many re-cent researches [1–8] have adopted this trend and carried outtheir work on naturally occurring substances. Promising resultswere obtained in previous work in this field. It was reported thatKhillah extract inhibits steel corrosion in HCl solution with in-hibition efficiency as high as 99% [1], while opuntia extractinhibits the corrosion of aluminum in the same acid with effi-ciency of about 96% [8].

This work is devoted to examining the aqueous extract ofolive (Olea europaea L.) leaves as an inhibitor for corrosionof C-steel in 2.0 M HCl solution. Weight loss measurements,potentiostatic polarization, and cyclic voltammetry were usedin the study.

A.Y. El-Etre / Journal of Colloid and Interface Science 314 (2007) 578–583 579

2. Experimental details

ASTM A573 Grade 70 low carbon steel was used inthe present study. The chemical composition of this steel is0.27% C, 0.85% Mn, 0.035% P, 0.035% S, 0.15% Si, and theremainder is iron. The steel specimens were taken from thetanks of Cairo Co. for petroleum refining.

Coupons of steel with dimensions (1, 1, 0.5 cm) and exposedsurface area 4 cm2 were used for weight loss measurements.For potentiostatic polarization experiments, a cylindrical rod ofsteel embedded in araldite with exposed surface area 0.38 cm2

was used. The electrodes were polished with different gradesof emery papers, degreased with acetone, and rinsed in distilledwater before they were inserted into the test solution.

All chemicals used for preparing the test solutions were ofanalytical grade and the experiments were carried out at roomtemperature, 25 ± 1 ◦C.

Dry olive leaves were finely divided and extracted in boiledwater for 3 h. The filtrate was evaporated in a steam bath andthe solid residue was left overnight in open air for completedryness. A stock solution was prepared, by weight, from thecollected solid and used to prepare the desired concentrationsby dilution.

For weight loss measurements, the steel coupons were lefthanging in the test solution for 1 day at 25 ± 1 ◦C before theirloss of weight was recorded. The corrosion rate was calculated,in milligrams per square centimeter per hour (mg/(cm2 h)), onthe basis of the apparent surface area. The inhibition efficiencycalculations were based on the weight loss measurements at theend of the whole exposure period. The results of the weight lossexperiments are the mean of three runs, each with a fresh steelsheet and a fresh acid solution. The percentage of inhibitionefficiency was calculated using the equation

(1)IE =[(W − Wi)

W

]× 100,

where W and Wi are the corrosion rates of the steel coupons inthe absence and presence of olive leaf extract, respectively.

Potentiostatic polarization and cyclic voltammetry experi-ments were carried out using a PS remote potentiostat with PS6software for calculation of corrosion parameters (corrosion cur-rent density, corrosion potential, and Tafel constants) as well ascurrent integration for charge density calculations. The corro-sion parameters were calculated from the intercept of the anodicand cathodic Tafel lines. A three-compartment cell with a sat-urated calomel reference electrode (SCE) and a platinum foilauxiliary electrode was used. The inhibition efficiency IE wascalculated using the equation

(2)IE =[(I − Ii)

I

]× 100,

where I and Ii are the corrosion current densities in free andinhibited acid, respectively.

3. Results and discussion

3.1. Weight loss measurements

The data of Table 1 represent the results obtained fromweight loss experiments. Inspection of the table reveals that therate of carbon steel corrosion is greatly reduced upon the ad-dition of olive leaf extract. This behavior reflects the inhibitiveeffect of the extract toward the acid corrosion of the steel. Theinhibition efficiency increases as the added extract concentra-tion is increased. It reaches 91% for 900 ppm of the addedextract.

3.2. Adsorption behavior

The inhibitive action of olive extract toward the acid corro-sion of steel could be attributed to the adsorption of its compo-nents onto the steel surface. The adsorbed layer acts as a barrierbetween the steel surface and the aggressive solution, leadingto a decrease in the corrosion rate. It follows that the inhibi-tion efficiency (IE) is directly proportional to the fraction of thesurface covered by the adsorbed molecules (θ ). Therefore, (θ )is calculated using the relation θ = IE/100, and the calculatedvalues are presented in Table 1. The mode of variation of (θ )with the extract concentration specifies the adsorption isothermthat describes the system.

The aqueous extract of olive (Olea europaea L.) leaf con-tains polyphenolic compounds with an antioxidation poten-tial, so it was used historically in folk remedies [9–11].Among these phenolics, the major constituents of the leafextract are oleuropein (C25H32O13) and hydroxytyrosol (3,4-dihydroxyphenylethanol). Oleuropein is readily hydrolyzedto hydroxytyrosol and elenolic acid. The other polyphenolspresent include tyrosol, oleuropein aglycone, and gallic acid.The chemical structures of polyphenols are represented in Fig. 1Because oleuropein is hydrolyzed into elenolic acid and hy-droxytyrosol [12,13], the latter may play the major role inthe inhibition process. Thus, the extract concentration is rep-resented as hydroxytyrosol in the adsorption isotherm diagram.This representation enables the calculation of an approximatevalue of the molar free energy of adsorption. When extractconcentration, as molar hydroxytyrosol (C), is plotted against(C/θ), a straight line with almost unit slope is obtained, asshown in Fig. 2. This behavior suggests that the olive com-pounds adsorbed onto the steel surface following the Langmuir

Table 1Corrosion behavior of C-steel in free and inhibited 2 M HCl solutions as re-vealed from weight loss experiments

Extract conc.(ppm)

Extract conc.(as molar hydroxytyrosol)

Wt. loss(mg/(cm2 h))

θ

– – 0.56 –50 0.00032 0.24 0.57

200 0.00128 0.18 0.68500 0.0032 0.07 0.87700 0.0045 0.06 0.90900 0.0058 0.05 0.91

580 A.Y. El-Etre / Journal of Colloid and Interface Science 314 (2007) 578–583

Fig. 1. Structure of hydroxytyrosol and oleuropein.

Fig. 2. Langmuir isotherm for adsorption of olive leaf extract components, rep-resented as hydroxytyrosol, onto steel surface in 2 M HCl solution.

adsorption isotherm. According to the Langmuir adsorptionisotherm, there are no interaction forces existing between theadsorbed molecules, and the energy of adsorption is indepen-dent of the surface coverage (θ ).

The Langmuir adsorption isotherm could be represented us-ing the equation [14]

(3)C

θ= 1

k+ C,

where K is the adsorption constant and

(4)lnk = ln1

55.5− �G◦

ads

RT,

where �G◦ads is the standard free energy of adsorption, where

one molecule of water is replaced by one molecule of inhibitorand the numerical value (1/55.5) in the equation stands for themolarity of water. The calculated value of free energy of ad-sorption was found to be −28.74 kJ/mol. The negative signindicates that the adsorption of olive components onto the steelsurface is a spontaneous process. It is well known that values of�Gads on the order of −20 kJ/mol or lower indicate a physicaladsorption, while those of −40 kJ/mol or higher involve chargesharing or a transfer from the inhibitor molecules to the metalsurface to form a coordinate type of bond [15]. The obtainedvalue of �Gads suggests a strong physical adsorption of oliveleaf extract components onto the steel surface in HCl solution.

Fig. 3. Relation between inhibition efficiency and olive leaf extract concentra-tion.

The phenolic compounds adsorb on the steel surface throughthe lone pairs of electrons of the oxygen atoms forming a cover-ing film. Inspection of the phenolic structures in Fig. 1 revealsthat the oxygen atoms almost surround the aromatic rings of thephenolics. This arrangement of the oxygen atoms may lead tothe conclusion that the phenolic compound is forced to be ad-sorbed horizontally onto the steel surface. This adsorption givesrise to a large covered surface area with a small number of ad-sorbed molecules. Therefore, high inhibition efficiency couldbe obtained by relatively low concentrations of the extract. Atthe same time, the increase of extract concentration above acertain value has little effect on the inhibition efficiency. Thisconclusion is confirmed by the fact that the inhibition efficiencydoes not increase linearly with extract concentration, as shownin Fig. 3. As revealed from the figure, the inhibition efficiencymaintained almost constant values for extract concentrationsabove 500 ppm.

3.3. Polarization studies

The cathodic and anodic polarization curves of C-steel insolutions of 2 M HCl devoid of and containing different con-centrations of olive leaf extract were recorded at a scanning rateof 5 mV/s and are represented in Fig. 4. Inspection of the figurereveals that the polarization curves are shifted toward more neg-ative potentials and less current density upon addition of oliveextract. This result confirms the inhibitive action of the oliveextract toward acid corrosion of C-steel.

The corrosion parameters of C-steel in 2 M HCl solutionsdevoid of and containing different concentrations of olive leafextract were calculated from polarization curves and presentedin Table 2. Inspection of the data of the table revealed thatthe corrosion potential shifts to more negative values with in-creased concentrations of olive extract. Moreover, the corrosioncurrent density decreases markedly on addition of the extract.The magnitude of this decrement increases as the concentrationof the extract is increased. Therefore, the inhibition efficiencyincreases with increasing extract concentration. Further inspec-tion of the table reveals that both the anodic and cathodic Tafelconstants increase with increasing extract concentration. This

A.Y. El-Etre / Journal of Colloid and Interface Science 314 (2007) 578–583 581

Fig. 4. Anodic and cathodic polarization curves of steel in solutions of 2 M HCl free and inhibited by different concentrations of olive leaf extract.

Fig. 5. Cyclic voltammetry of C-steel in 2 M HCl solution free and inhibited by different concentrations of olive leaf extract. Scanning rate = 10 mV/s.

Table 2Corrosion parameters of C-steel in solutions of 2 M HCl, free and inhibited bydifferent concentrations of olive leaf extract, at 298 K

Inh. conc.(ppm)

Ecorr(mV)

Icorr(mA/cm2)

IE% βa −βc

– −468 0.42 – 110 20450 −510 0.12 71 121 254

200 −512 0.07 83 145 271500 −516 0.05 88 153 308700 −520 0.04 90 178 318900 −523 0.03 93 191 342

result suggests that the presence of the extract affects the an-odic dissolution of steel as well as the cathodic reduction ofhydrogen ions. Therefore, it could be concluded that the mole-cules of extract absorb onto both anodic and cathodic sites of

the steel surface. This behavior indicates that the extract acts asa mixed inhibitor.

3.4. Cyclic voltammetry

The cyclic voltammograms of C-steel in solutions of 2 MHCl devoid of and containing different concentrations of oliveleaf extract were recorded and are represented in Fig. 5. Thepotential was started at −1900 mV and swept in the positive di-rection up to oxygen evolution at a scanning rate of 10 mV/s.Inspection of Fig. 5 reveals that the anodic branches of the steelin free and inhibited acid solutions exhibit a very narrow pas-sive region with no anodic dissolution peaks. The transpassiveregion and the onset of oxygen evolution start in both solutionsat almost the same potential; about −350 mV.

582 A.Y. El-Etre / Journal of Colloid and Interface Science 314 (2007) 578–583

Table 3Corrosion parameters of C-steel in solutions of 2 M HCl free and inhibited by500 ppm of olive leaf extract at different temperatures

Temp.(K)

Solution Ecorr(mV)

Icorr(mA/cm2)

IE% βa −βc

298 Free −390 0.42 – 88 129Inhibited −516 0.05 88 153 308

313 Free −395 0.57 – 81 121Inhibited −439 0.16 72 66 168

323 Free −400 0.74 – 69 119Inhibited −417 0.32 57 78 140

333 Free −412 0.83 – 55 115Inhibited −428 0.63 24 76 136

343 Free −392 0.99 – 51 110Inhibited −401 0.92 7 63 120

Further inspection of the curves of Fig. 5 reveals that uponthe onset of the transpassive region the current still rises up untilthe potential is reversed, in the case of steel in free acid solu-tion. On the other hand, the current of the transpassive regiondecreases in the presence of the extract, forming a broad anodicpeak. The formation of such a peak could be attributed to theadsorption of the extract components on the steel surface. Thevalues of charge density of the transpassive region were calcu-lated for the three tested solutions and found to be 109, 102,and 99 C for free, 50, and 500 ppm of extract, respectively. Itis obvious that the charge density decreases with increasing ex-tract concentration. This result could be interpreted in view ofthe adsorption of extract components onto the steel surface. Theadsorbed molecules form a barrier for charge and mass transfer.

3.5. Effect of temperature

The effect of temperature on the corrosion of C-steel in freeand inhibited 2 M HCl solutions was studied using potentio-static polarization in the range of 25–70 ◦C. The acid solutionswere inhibited by addition of 500 ppm of olive leaf extract. Thecorrosion parameters calculated from the polarization curvesare given in Table 3. Inspection of Table 3 reveals that thecorrosion potential shifted to a more active potential as the tem-perature was increased. Moreover, the corrosion rate of steel inboth free and inhibited acid media increased as the temperaturewas increased. However, the inhibition efficiency of the oliveextract decreases markedly with increasing temperature. Thisresult supports the idea that the adsorption of extract compo-nents onto the steel surface is physical in nature. Thus, as thetemperature increases, the number of adsorbed molecules de-creases, leading to a decrease in the inhibition efficiency.

The activation energies of the corrosion process in free andinhibited acid were calculated using the Arrhenius equation[16],

(5)k = A expEa

RT,

where Ea is the activation energy, A is the frequency factor,T is the absolute temperature, R is the gas constant, and K

is the rate constant, which is directly proportion to the corro-sion current (Icorr). Plotting log I versus 1/T gives a straight

Fig. 6. Arrhenius plot for steel corrosion in 2 M HCl solution free and inhibitedby 500 ppm of olive leaf extract.

line, as revealed by Fig. 6. The values of activation energy cal-culated using the lines of Fig. 6 are 3.02 and 10.41 kJ/molfor free and inhibited acid solutions, respectively. The obtainedresults suggest that olive leaf extract inhibits the corrosion re-action by increasing its activation energy. This could be doneby adsorption onto the steel surface, making a barrier to massand charge transfer. However, such types of inhibitors performgood inhibition at ordinary temperature, with considerable lossin inhibition efficiency at elevated temperatures [17].

4. Conclusions

– Olive leaf extract acts as a good corrosion inhibitor forC-steel in 2 M HCl solution.

– The inhibition action of the extract was attributed to thephysical adsorption of its phenolic compounds, mainlyoleuropein and hydroxytyrosol, onto the steel surface.

– The extract acts as a mixed inhibitor.– The inhibition efficiency decreases with increasing temper-

ature.– The presence of extract increases the activation energy of

the corrosion process.– The presence of extract decreases the charge density in the

transpassive region.

References

[1] A.Y. El-Etre, Appl. Surf. Sci. 252 (2006) 8521.[2] A. Bouyanzer, B. Hammouti, L. Majidi, Mater. Lett. 60 (2006) 2840.[3] M. Benabdellah, M. Benkaddour, B. Hammouti, M. Bendahhou,

A. Aouniti, Appl. Surf. Sci. 252 (2006) 6212.[4] E. Chaieb, A. Bouyanzer, B. Hammouti, M. Benkaddour, Appl. Surf.

Sci. 246 (2005) 199.[5] B. Müller, Corros. Sci. 44 (2002) 1583.[6] Y. Li, P. Zhao, Q. Liang, B. Hou, Appl. Surf. Sci. 252 (2005) 1245.[7] A.Y. El-Etre, M. Abdallah, Z.E. El-Tantawy, Corros. Sci. 47 (2005) 385.[8] A.Y. El-Etre, Corros. Sci. 45 (2003) 2485.[9] B. Müler, M. Skerget, P. Kotnik, M. Hadolin, A. Rižner Hra, M. Simoni,

Z. Knez, Food Chem. 89 (2005) 191.[10] O. Benavente-Garcõâ, J. Castillo, J. Lorente, A. Ortunä, J.A. Del Rio,

Food Chem. 68 (2000) 457.[11] M.G. Soni, G.A. Burdock, M.S. Christian, C.M. Bitler, R. Crea, Food

Chem. Toxicol. 44 (2006) 903.[12] V. Marsilio, B. Lanza, N. Pozzi, J. Am. Oil Chem. Soc. 73 (1996)

593.

A.Y. El-Etre / Journal of Colloid and Interface Science 314 (2007) 578–583 583

[13] F. Capozzi, A. Piperno, N. Uccella, J. Agric. Food Chem. 48 (2000) 1623.[14] I.O’M. Bockris, D.A. Swinkls, J. Electrochem. Soc. 111 (1964) 736.[15] M. Donahue, K. Nobe, J. Electrochem. Soc. 112 (1965) 886.

[16] R.T. Vashi, V.A. Champaneri, Ind. J. Chem. Technol. 4 (1997) 180.[17] I.K. Putilova, S.A. Balezin, Y.P. Barasanik, Metallic Corrosion Inhibitors,

Pergamon, Oxford, 1960, p. 30.